This invention relates to a strip antenna whose polarization characteristic can be changed.
A polarization antenna is generally demanded to have its polarization characteristic electrically changed. FIG. 1 shows a linearly polarized strip antenna capable of changing the direction of polarization. This strip antenna comprises a dielectric substrate 10 whose backside is fitted with a ground conducting film, power supply circuit 12 including a strip line which is provided on the dielectric substrate 10 and is formed of a dielectric film and a linearly polarized radiator 14 which is formed of a rectangular conductive film. The power supply circuit 12 is arranged as follows. A strip line 16 is divided into two paths by a power divider 18. One path 20 is connected to the center of one side of a radiation element 14. The other path 22 is connected to the anode of a diode 28 and the cathode of a diode 30 through a capacitor 24 and strip line 26. The cathode of the diode 28 is connected to the cathode of a diode 34 through a strip line 32. The anode of the diode 30 is connected to the anode of a diode 38 through a strip line 36. The anode of the diode 34 and the cathode of the diode 38 are connected through a strip line 40 and capacitor 42 to one side of the radiation element 14 which lies adjacent to that side to which the one path 20 is connected. In this case, two paths connecting the radiation element 14 and divider 18 together are chosen to have an equal electric length. The power divider 18 divides the power supplied to the strip line 16 so that the divided power components have the same phase and amplitude. The power running through the strip lines 32 and 36 are arranged to have the same amplitude, but to be displaced 180.degree. from each other in respect of phase. The strip lines 26 and 40 are connected to a bias terminal 48 through the corresponding low path filters 44 and 46. The strip lines 32 and 36 are connected to a ground terminal through the corresponding low path filters 50 and 52. The capacitors 24 and 42 prevent the DC bias conducted to the diodes 28, 30, 34 and 38 from being diverted to any other circuit section. Conversely, the low pass filters 44, 46, 50 and 52 allow for the passage of the DC component, but prevent high frequency current delivered to the strip lines from being conducted to the bias terminal 48 or ground terminal.
The operation of the conventional strip antenna shown in FIG. 1 is now given. Where a positive bias voltage is impressed on the bias terminal 48, then the diodes 34 and 28 are rendered conductive, and the diodes 30 and 38 are rendered nonconductive. At this time, high frequency current components supplied to the radiation element 14 through the two divided paths have the same phase and same amplitude. As shown in FIG. 2A, therefore, current runs in the directions indicated by broken lines. The composite current flows in the direction of the indicated solid line. Where a negative bias voltage is impressed on the bias terminal 48, then the diodes 38 and 30 are rendered conductive, and the diodes 28 and 34 are rendered nonconductive. At this time, the current components delivered to the radiation element 14 through the two divided paths have the opposite phases and same amplitude. In the radiation element 14, therefore, two currents flow in the directions of broken lines shown in FIG. 2B, and the composite current runs in the direction of the indicated solid line. As described above, the direction of the current conducted through the radiation element 14 is displaced 90.degree. in accordance with the polarity of the voltage impressed on the bias terminal 48. As a result, a radiated electromagnetic wave is polarized in a direction displaced 90.degree..
However, the conventional linearly polarized antenna arranged as described above has the following drawbacks. The current components supplied to the radiation element 14 through the two divided paths are demanded to have the same phase or opposite phases. Since, however, diodes are provided in one of the two divided paths, the phase relationship can not be accurately controlled. Further, it is difficult to let the two divided current components have exactly the same phase in the divider 18. Consequently the direction of polarization is not changed to an extent of accurately 90.degree., thereby probably leading to a decline in cross polarization discrimination. Further, errors tend to occur in the amplitudes of the two divided current components due to errors in the lengths of the strip lines 32 and 36. With the conventional strip antenna arranged as described above, a large number of elements have to be provided in the power supply circuit, causing the strip antenna to occupy a larger space and increase in cost. Where an array antenna in particular is constructed, an increase in the area occupied by the antenna restricts the location of a radiation element. The above-mentioned drawbacks occurred not only in the linearly polarized antenna, but also in the circularly or elliptically polarized antenna applied to date.